Direct torque control (DTC) is a method for controlling the torque and the electromagnetic flux in rotating electromagnetic machines and motors, for example, in the medium-voltage range. A typical example of such a controller can be found, for example, in the drive of the ACS 6000 by ABB. Over the last 10 years, DTC has demonstrated a high degree of reliability, robustness and performance.
However, the switching losses, which are produced due to semiconductors being switched by the controller and which represent a large proportion or even the majority of the total losses of the drive, can be considerable in the case of DTC. Therefore, ways of reducing these switching losses are sought. It is known that such a reduction in switching losses can be achieved by so-called model predictive direct torque control (MPDTC), which can be based on a (mathematical) model of the drive. U.S. Pat. No. 7,256,561 and EP 1 670 135 describe such a method.
In addition to the minimization of the converter switching losses, DTC and MPDTC can also be geared towards keeping the three output variables of the rotating electrical machine, namely the electromagnetic torque, the variable of the stator flux and the neutral point potential(s) or mid-point potential(s), within predetermined (hysteresis) limits.
MPDTC is based on a control algorithm which can include a mathematical model that is matched to the converter to be switched and to the machine or motor connected thereto. In this case, the permissible switching sequences of the converter over a specific switching horizon (e.g., period) are listed and the corresponding torque, stator flux and intermediate-circuit mid-point potential trajectories are calculated using an internal model of the converter and the machine. There are also converters with more than one neutral point. There are also converters which have other variables that need to be regulated, under certain circumstances. In this case, these trajectories are extrapolated until a limit for the respective output variables is reached. The permissible switching sequences are evaluated using a predetermined quality criterion. This quality criterion maps, for example, the switching losses or the switching frequency of the converter. In a last step, an optimal switching sequence is determined which minimizes this quality criterion, e.g., a quality criterion which provides minimum switching losses or a minimum switching frequency, for example. The aforementioned steps aforementioned are generally implemented in any control cycle and, of the optimal switching sequence, only the first step (e.g., the first switching transition or switching state) is used for controlling the power semiconductors of the converter.